From hydration repulsion to dry adhesion between asymmetric hydrophilic and hydrophobic surfaces

Abstract

Using all-atom molecular dynamics (MD) simulations at constant water chemical potential in combination with basic theoretical arguments, we study hydration-induced interactions between two overall chargeneutral yet polar planar surfaces with different wetting properties. Whether the water film between the two surfaces becomes unstable below a threshold separation and cavitation gives rise to long-range attraction, depends on the sum of the two individual surface contact angles. Consequently, cavitation-induced attraction also occurs for a mildly hydrophilic surface interacting with a very hydrophobic surface. If both surfaces are very hydrophilic, hydration repulsion dominates at small separations and direct attractive force contribution can-if strong enough-give rise to wet adhesion in this case. In between the regimes of cavitation-induced attraction and hydration repulsion we find a narrow range of contact angle combinations where the surfaces adhere at contact in the absence of cavitation. This dry adhesion regime is driven by direct surface-surface interactions. We derive simple laws for the cavitation transition as well as for the transition between hydration repulsion and dry adhesion, which favorably compare with simulation results in a generic adhesion state diagram as a function of the two surface contact angles.A ccording to the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, the interaction between hydrated surfaces is given by the sum of van der Waals (vdW) and screened electrostatic interactions (1). Although this approach works well in many situations, subnanometer resolved force measurements between individual surfaces demonstrated that additional watermediated interactions are dominant at small separations and depend crucially on the polarity or wetting properties of the surfaces (2-4). Understanding these solvation-induced interactions is still a central issue in all fields concerned with forces between surfaces, colloids, and macromolecular aggregates in water.As is well known, the water film between two hydrophobic surfaces, characterized by water contact angles θ > 90°, becomes freeenergetically unstable below a critical distance and in equilibrium cavitation leads to vapor bubble-induced long-ranged attraction (5-7). Conversely, polar and overall neutral surfaces, characterized by small or vanishing contact angles, exhibit pronounced shortranged repulsive forces, which decay exponentially with a characteristic length in the subnanometer range (8-11). These so-called hydration forces arise from the complex interplay of surface group configurational degrees of freedom, desorption of hydration water from polar surface groups, and ordering of the intersurface water film (4). The understanding of hydration forces has recently been advanced by computer simulations that include explicit water molecules (12)(13)(14).In the absence of direct surface-surface interactions, the transition between cavitation-induced attraction and hydration repulsion should coincide with the contact angle char...

Enhanced flux in direct contact membrane distillation using superhydrophobic PVDF nanofibre membranes embedded with organically modified SiO2 nanoparticles

“…A change in the LEP also is caused by cavitation induced by the hydrophobic nature of the SiO 2 NP‐embedded PVDF nanofibre membrane and the water polarity. Water becomes energetically unstable on hydrophobic surfaces, which increases with increasing membrane–water contact angle . This phenomenon results in the formation of water bubbles which consequently block the pores of the membrane, thus requiring greater pressure to drive the water across the membrane.…”

“…The membranes M3, M4 and M5 are characterized by a contact angle higher than 150° which promotes water passage in vapour state, thus preventing membrane wetting. This minimal wetting reduces the passage of water vapour, which subsequently minimizes the water vapour flux . Although M1 and M2 are characterized by larger pore sizes and higher porosity (i.e.…”

“…Water becomes energetically unstable on hydrophobic surfaces, which increases with increasing membrane-water contact angle. 51 This phenomenon results in the formation of water bubbles which consequently block the pores of the membrane, thus requiring greater pressure to drive the water across the membrane. This increase in LEP has been reported previously during the incorporation of additives (e.g.…”

“…This minimal wetting reduces the passage of water vapour, which subsequently minimizes the water vapour flux. 51 Although M1 and M2 are characterized by larger pore sizes and higher porosity (i.e. properties required for the enhancement of water flux) than those of M3, M4 and M5, their thickness would be another factor associated with their lower fluxes.…”

The structural origin of hydration repulsive force

“…However, when two surfaces or particles approach closer than a few nanometres, the repulsive interactions (hydration force) between two solid surfaces in a liquid medium fail to be accounted for by DLVO theory. Therefore, many works have been carried out to investigate the origin of hydration force [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33].…”